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Proximity Swipe Card Repair 



Make] Projects 



Proximity Swipe Card Repair 

Written By: Graham Cattley 



TOOLS: 


PARTS: 


Craft knifed) 


Proximity card (1) 


Jeweler's loupe (1) 


Proximity card (1) 


Locking tweezers (1) 


Rubbina Alcohol (1) 


or "third hand" tool 




• Raqs (1) 




Soldering iron (1) 





SUMMARY 

It all started one day at work. I had used my swipe card to get through the front doors, and 
without thinking too much, I put it in my back pocket. Later that day I sat down — crunch! 
The card had cracked from side to side. I wrapped it in tape to hold it together, and it still 
worked for two weeks, but then something in it failed. I tailgated people in and out of the 
building that day. 

I had paid my company a $100 deposit on that pass card, and that night I decided I wasn't 
going to give it up by asking HR for a new one. I was determined to fix it myself. 



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Proximity Swipe Card Repair 



Step 1 — Background 

• It all started one day at work. I had used my swipe card to get through the front doors, and 
without thinking too much, I put it in my back pocket. Later that day I sat down — crunch! 
The card had cracked from side to side. I wrapped it in tape to hold it together, and it still 
worked for two weeks, but then something in it failed. I tailgated people in and out of the 
building that day. 

• I had paid my company a $100 deposit on that pass card, and that night I decided I wasn't 
going to give it up by asking HR for a new one. I was determined to fix it myself. A bit of 
research showed that these pass cards are often called "proximity cards," and operate in a 
manner similar to the tiny RFID tags found inside the new U.S. passports and even pet 
dogs. 

• See Step 6, "How Does It Work?" for a basic explanation of their operation. 



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Proximity Swipe Card Repair 



Step 2 




• Bending the card slightly to widen 
the crack revealed a coil of fine 
copper wire inside, and when I 
dismantled the card completely, I 
was surprised to find that its plastic 
shell contained not only a coil but 
also a small integrated circuit (IC) 
on a printed circuit board (PCB). 
Even more surprisingly, it had no 
battery! 

• I carefully removed some of the 
outside shell to reveal more, and 
then exposed the coil and PCB 
completely. 

• With the aid of a jeweler's loupe, I 
could see that the wire coil had 
broken —in many places, 
unfortunately. It would have been a 
major job just to identify the correct 
wires to solder back together, let 
alone actually doing it! Plan B was 
to wind my own coil, but back in 
the depths of my mind were the 
makings of Plan C: replacing the 
coil with one from another card. 



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Proximity Swipe Card Repair 



Step 3 — Extract the Donor Coil 




• After 20 minutes pawing through 
one of my junk boxes in the attic, I 
found an old pass card from a 
previous employer. It carried a 
different code, so it wouldn't work 
at my current office, but its coil 
was intact. What's more, the heat 
in the attic had dried out the glue — 
the card practically fell apart in my 
hands. Plan C was the way to go! 

• I used a sharp craft knife to pry 
away the old glue that held the coil 
and mini PCB to the plastic shell. I 
took great care, as I couldn't afford 
any breakages. After 10 minutes, 
the salvaged coil was free. 

• I unsoldered the donor coil from its 
original PCB using a fine-tipped 
soldering iron, holding the PCB with 
a pair of locking tweezers. This 
was easy, as I didn't have to worry 
about overheating and possibly 
damaging the old IC. After 
removing the coil, I tested its 
continuity with a multimeter. 
Success! The coil was intact. 



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Proximity Swipe Card Repair 



Step 4 — Attach the New Coil 

• The tricky part was attaching the new PCB. This time, I had to be careful to keep the iron 
on the joint for as little time as possible, since the PCB was small and could quickly 
overheat, killing it forever. 

• Things went well, though, and because the ends of the coil were already tinned where I 
removed them from the old board, the joints were easy to make. The coil from my old card 
was now connected to the mini PCB from my new card. 



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Proximity Swipe Card Repair 



Step 5 — Seal It Up 




• Now it was time to look at the 
housing. My old card's shell was 
filled with dried-out glue and 
covered with ground-in grime. 
Fifteen minutes later the case was 
clean, thanks to some isopropyl 
alcohol, a soft cotton rag, and a bit 
of elbow grease. 

• The last step was to position the 
coil and PCB in the old case and 
hold it in place with a few drops of 
glue. The card's thin plastic 
backing had suffered from the 
cleaning process, so I elected to 
replace it with a piece of clear, 
self-adhesive book film trimmed in 
place with a craft knife. (White 
book film would have been fine, but 
with clear film, the card made a 
great talking point with my co- 
workers.) The results looked good, 
but the acid test would have to wait 
until the next morning at work. 

• At 8:36 a.m. I waltzed up to the 
front door and waved my pass at 
the security panel. "Click!" went the 
door. "Woo hoo!" I shouted. The 
guys in HR couldn't understand 
why I was smiling so much that 
day. 



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Proximity Swipe Card Repair 



Step 6 — RFID Tag: How Does It Work? 

• There is a lot of misinformation out there about how RFID works. Here's a typical 
explanation: 

• The reading unit (attached to the door) transmits radio energy. The card receives this 
energy, converts it to electrical energy, and then uses it to transmit its serial number back 
to the reading unit, which recognizes the card and opens the door. 

• While this is neat and simple, it's also wrong. Yes, the card's coil antenna absorbs the RF 
energy, and this is used to power the chip, but technically speaking, the card doesn't 
transmit anything at all! 

• Without going into the gory details (look up "load modulation" if you want them), you can 
better explain RFID with the following analogy: 

• Let's say you're out on a boat, and you want to use a mirror to send information to a 
lighthouse. You can encode it in a binary format and then transmit one bit each time that 
the lighthouse beam sweeps by, where reflect means 1 and not reflect means 0. 

• Security pass cards use the same principle, but instead of one sweep every 10 seconds or 
so, the reader transmits at 125kHz. The card's chip communicates its data by selectively 
shorting out its coil over successive cycles of the 125kHz transmission. A shorted coil 
doesn't absorb any of the RF energy, while a non-shorted coil does, so the reader then 
distinguishes 1 from by measuring the peak voltage on its antenna to see if it's high or 
low. In this way, the card communicates its 24-bit serial number, along with synchronizing 
and checksum data. 

This project first appeared in MAKE Volume 19 . 
Related Posts on Make: Online: 

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http://blog.makezine.com/archive/2010/05... 
Autonomous Snow Shovel Robot 
blog.makezine.com/archive/2009/01/autonomous_snow_shovel_robot.html 

last generated on 2012-10-31 11:45:42 PM. 



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